Method and apparatus for deposition of materials by thermal decomposition



Aug. 3, 1954 H. NACK 2,635,535

METHOD AND APPARATUS FOR DEPOSITION OF MATERIALS BY THERMALDECOMPOSITION Filed Feb. 1', 1951 INVENTOR HERMAN NACK Wm ,1 mm

ATTORNEYS Patented Aug. 3, 1954 METHOD AND APPARAT OF MATERIALS BY T'SITION US FOR DEPOSITION HERMAL DECOMPO- Hermjan Naek, Troy, Ohio,assignor to The Gommonweaxlth- Engineering Company of Ohio, Dayton,Ohio, a corporation of Ohio Application February 1, 1951, Serial No.208,934

4 Claims.

This .invention relates to the art of deposition of metals. Moreparticularly, it relates to the plating of metals from the gaseous.state, and to the apparatus for carrying out the saidprocess.

The deposition of thinfilms of protective metal, .such as nickel,cobalt, tungsten and molybdenum, or their alloys, on metallic bases hasbeen accomplished by enclosing an object to be plated in a chamber,filling the chamber with decomposable metal-bearing gas, heating theobject to be plated to the decomposition temperature of the said gas,and allowing the metal-bearingg-as-to contact the heated plate and bedeposited thereon.

Considerable :diificulty has been experienced in securing dense filmdeposits withsuch a method 1 and various expedients have been devised'toovercome the inherent limitations of the structure of thisapparatus.These limitations stem from two primary causes. The first of these isthat the gaseous decomposition may take place prematurely, that is,before the gas contacts the article .to be plated, thus permitting metalparticles to fall upon the surface to be plated resulting in roughnessof the plate coating.

Secondly, an interracial resistance between the advancing wave ofplating .gas and the surface of the object to be plated opposes thedecomposition directly on the 'free surface of the object.

The result of these effects of uncontrolled decomposition is theproduction of thin, porous, irregular coatings of deposited metal.While, as

noted above, means to effect the control of these undesirable phenomenahave been accomplished they require expert workmanship and specializedapparatus.

It is an object of this invention to provide a method by which the abovementioned disadvantages are overcome.

It is an important object of this invention to provide a novel methodfor gas plating by which a firmly adhering metal coating is obtained.

It is a principal object of this invention to provide a method of gasplating which yields coatings of increased density.

It is also an object of this invention to provide a metal coating whichwill withstand elevated chamber i More specifically, in the process ofinvention the application of an electrostatic field which has theworkpiece as one electrode thereof causes ionized gases to be directedto the workpiece at great velocity. While in heated gases the collisionsbetween molecules give rise to ionization to some degree, it isdesirable to increase this velocity of collision by means or theelectrostatic field and the presence of charged gases in the platingchamber by exposing the gases, both carrier and platin gas, toradioactive substances prior to their entry to the chamber.

It should also be noted that it is preferable to secure the secondelectrode of the electrostatic field to a conductive outlet portion ofthe chamber whereby the negatively charged gases of decomposition willbe attracted thereto and rapidly swept from the chamber. This mode ofoperation results in decreasing the resistance to the flow ofmetal-bearing vapors to the surface of the metal to be plated and thusprovides for intimate contact of the metal-bearing component with thesurface and gives rise to dense uniform non-porous deposits.

The nature and purpose of this invention has been indicated in a generalway and there follows a more detailed description of the preferredembodiments of the invention with reference to the accompanying drawingin which:

Figure 1 is a view partly in section of the apparatus of invention; and

Figure 2 is a View of apparatus utilized in carrying out one embodimentof the invention.

Referring to Figure 1, there is shown a charm her I having a gas inlet 2and an outlet 3 electrically insulated from the chamber. Mounted in thechamber is a heating unit 4 connected to a source of energy (not shown)by an insulated lead it extending through the wall 5 of the Supported onthe heating unit 4 by electrically insulating support members it is aworkpiece S to which there is electrically secured an electricallyconductive lead i passing through the wall 5 in electrically insulatedrelation therewith.

The connection between the workpiece 6 and the lead i may be throughmeans of any suitable contact arrangement, such as clamps, sockets, orthe like. Electrically connected to the vopposing end of the lead '3- isthe negative pole of a source of direct current, generally indicated at8. The positive pole of the source 8 is connected by a lead '9 to aconductive portion of outlet 3, thereby setting up an electrostaticfield between the inner end, of the outlet 3. and-the workpiece B. Theouter end of the outlet 3 is preferably of insulating material but insome instances may be conductive and in such a case is insulated fromground.

In operation metal-bearing gases together with a carrier gas are fed tothe chamber i through the inlet 2 from suitable containers or mixingchambers which are not shown since they are well known to the art andform no part of the present invention.

The entering gases due to their velocities and frequent collisions maybe ionized to a very slight extent and are attracted to the workpiece orcathode 6. The negatively charged cathode will receive the positivelycharged molecules, causing decomposition of the metal-bearing gases intometallic ions and negatively charged ions of the gases of decomposition.The metallic ions lose their charge to the cathode, resulting in a metaldeposit, while negatively charged decomposition products will berepelled with great force and at high velocity from the area of theplated surface.

These repelled ions move under the influence of the electrostatic fieldto the positively charged outlet of the chamber, and in so doing strikeor collite with un-ionized gases thereby contributing to the overallionization of the gas in the chamber. Accordin ly when in full operationthe chamber will contain large quantities of positively charged metallicions, and negatively charged gases of decomposition which may in theionized state or as charged molecules of carbon monoxide, carbondioxide, and so forth. The charged metallic ions moving at high velocitytowards the cathodic workpiece will deposit thereon in a fine, uniformdense coating, thus overcoming the defect noted hereinbeiore of particles settling slowly and forming a rough surface.

It is clear from the foregoing that while it is preferable to have theelectrostatic field applied between the workpiece and the outlet of thechamber that the only requirement for impinging the metallic positivelycharger. ions on the.

workpiece is that the workpiece be the cathode. The positive plate or"the electrostatic field may be any other suitable conductive portion ofthe apparatus.

The electrostatic voltage necessary to effect the novel plating processset forth will be a able factor dependent upon the distance between theplates of the field, the nature of plating gas, the pressure andtemperature of this gas, and so forth. Other conditions being equalfield strength will decrease as the distance from an electrodeincreases, and an electrostatic potential of 1000 volts applied to theplates will be decidedly effective at a distance of between plates.Where the anode and cathodic workpiece are separated by as much as 2feet approximately 200,909 volts may be required for maximumeffectiveness. However, as been noted, the gas pressure and the natureof the gas may require alteration in these conditions.

The heater unit t is utilized to raise the ternperature of the cathodeworkpiece to accomplish complete decomposition of the metal hearing' gasat the workpiece surface. This temperature is a variable factordependent upon the plating gas used, and in the case of nickel carbonylgas'should be in excess of 180 C.

Base materials which may be plated in the foregoing manner are steel,copper, aluminum, cast iron, brass, magnesium, and the like.

The process and apparatus of invention are applicable for plating withall metals which form gaseous carbonyls, such as chromium, iron,tungsten, cobalt, molybdenum, tellurium, rhenium and nickel. Thecarbonyl of each of these metals has a temperature at whichdecomposition is complete, although some decomposition takes place atlower temperatures. The adverse eiiects of such low temperaturedecomposition are overcome by the high velocity attained by theparticles in the electrostatic field.

In the case of tungsten, nickel, chromium and iron carbonyls it ispreferred to maintain the temperature of the workpiece in the range of350 F. to 425 F., although temperatures below and above this range maybe utilized and good plating accomplished. The use of the electrostaticfield with the. attendant high velocity of the particles permits areduction in the temperature of the workpiece if other conditionsrequire it.

In Figure 2 there is shown a tubular member ill having an inlet H andoutlet l2 and containing radioactive material, such as uranium orradium, The function of the member ill is to effect a preliminaryionization of either the car rier gas which enters the plating chamberwith the metal carbonyl, or to efiect preliminary ioniaction of both thecarrier gas and the carbonyl. For this latter purpose outlet 52 ofmember It! may be connected directly to inlet 2 of chamber l.

The eiiect of the action of the radioactive material is to cause asplitting of the carbonyl molecule M (CO-i) into positively charged Mions and negatively charged (CO4) ions. Since the metallic ions are nowdissociated from the heavier (CO4) ions the velocity imparted to them bythe electrostatic field, before striking the workpiece, will beconsiderably greater than had. dissociation taken place closer to theobject to be plated.

The charged carrier gas serves to effectively attract the gases ofdecomposition from the area of the plating surface, thus further tendingto break down the interfacial resistance between the wave of plating gasand the surface of the cathodic workpiece.

Carrier gases useful for the above purpose are nitrogen, helium,hydrogen, carbon dioxide and other inert media. Hydrogen it may be notedis particularly useful since it effects a reducing action and mayaccordingly be advantageously used under conditions where iron rustmight develop and inhibit the process.

While the above embodiments of the invention have particularly set forththe use of carbonyls, it will be understood that other metal bearinggaseous compounds such as metal hydrides, metal alkyls, metal halidesand the nitroxyls such as that of copper, the nitroxyl carbonyls and thecarbonyl halogens fall within the scope of this invention and may beused with equal facility.

Cleaning of the workpiece preparatory to coating may be effected by anyof the means well known to the art, such as acid, alkali, orelectrochemical treatment, the only requirement being that a clean metalsurface be provided for the reception of the deposited metal to attainthe maximum benefit from the novel method of deposition.

It will be understood that this invention is susceptible to modificationin order to adopt it to different usages and conditions and,accordingly, it is desired to comprehend such modifications within thisinvention as may fall within the scope of the appended claims.

I claim:

1. An apparatus for coating an electrically conductive workpiececomprising a chamber having an inlet and an outlet, a support for saidworkpiece in said chamber, means for creating a high voltage electricalfield Within said chamber and having anode and cathode terminalsdisposed therein, said cathode being arranged for connection to saidworkpiece on said support, said anode being arranged at said outlet, andmeans comprising a tubular member having an inlet and outlet andcontaining radioactive material, said outlet of the tubular member beingconnected to the inlet of said chamber, means for heating said workpieceand means to supply a stream of gaseous metal carbonyl to the inlet ofsaid tubular member.

2. An apparatus as called for in claim 1 wherein the radioactivematerial is selected from the group consisting of radium and uranium.

3. A method of coating an electrically conductive workpiece comprisingthe steps of supporting the workpiece within a chamber, creating anelec- References Cited in the file of this patent UNITED STATES PATENTSNumber Name Date 1,820,878 Wyckolf Aug. 25, 1931 1,866,729 Spanner July12, 1932 2,332,309 Drummond Oct. 19, 1943 2,510,795 Blau et al June 6,1950 FOREIGN PATENTS Number Country Date Great Britain of 1947

3. A METHOD OF COATING AN ELECTRICALLY CONDUCTIVE WORKPIECE COMPRISINGTHE STEPS OF SUPPORTING THE WORKPIECE WITHIN A CHAMBER, CREATING ANELECTROSTATIC FIELD WITHIN SAID CHAMBER AND HAVING SAID WORKPIECECONNECTED AS THE CATHODE, CONDUCTING A GASEOUS METAL CARBONYL IN CONTACTWITH RADIOACTIVE MATERAIL AND THEREAFTER CONDUCTING SAID GASEOUS METALCARBONYL THROUGH SAID CHAMBER AND IN CONTACT WITH SAID WORKPIECE.